In a video surveillance system a fundamental element is the camera. Currently, analogue cameras are replaced by digital security cameras. In many cases, they are designed to use the existing digital communications networks, which entails a reduction in installation costs. That factor is the cause of the rapid increase in the number of that type of digital systems.
Other common devices in digital video surveillance systems are video servers. Their main function is to digitize the analogue video signal. Another typical characteristic is that they perform the functions of interface between the analogue cameras and the digital communications networks. This makes it possible to go gradually from an analogue video surveillance system to a digital one.
Central servers are the other devices that comprise, with those presented above, the set of basic elements of a digital video surveillance system. Their essential mission is the configuration of the system and the general control of the digital cameras and the video servers. Furthermore, it is very common that the video sequences obtained are filed in them.
These new digital video surveillance systems appear as a result of the enormous progress in the information technologies in recent decades. Parallel to their appearance, there arise numerous applications for the editing of still and video images. With them, it is not complicated to manage to alter an image, so that it is not possible to distinguish between an original and a false one. Furthermore, the number of potential manipulators has enormously increased since, due to Internet, those editing tools are at the reach of a large number of users. A consequence of the above is that, from the standpoint of authenticity, the validity of the digital still images and videos are increasingly questioned.
The marking of a digital document with a digital image is one of the solutions proposed to resolve the aforementioned problem. It is a set of techniques used to insert information in a digital document (image, video, audio, etc.). The introduction of the information is performed by modifying the original document (host) with the main restriction that the distortion produced by the marking is tolerable (in accordance with the application). One of its essential advantages is that the data inserted are linked to the host, hence no additional file is needed as occurs in the case of cryptography.
Several techniques are used for the classification of a specific technique of marking a digital document with a digital image. Two of the most important are the robustness and the need or not of the host to make the information extraction possible. On this last characteristic, it is said that a technique is blind if the host is not needed for the extraction of the information and not blind in the opposite case.
In video surveillance, an essential condition is that the original images are not needed to be able to extract the information, to avoid duplicating the storage capacity required. As a consequence, it is gathered that a watermarking technique for video surveillance systems should preferably be blind.
A technique for marking a digital document with a digital image is robust if the mark inserted resists alterations, either casual or intentional. In the opposite case we find the fragile techniques, which are those wherein the mark is corrupted after the slightest alteration. Fragile or semi-fragile techniques are used for digital content manipulation recognition, as they permit demonstrating the authenticity of the content analysing the integrity of the mark.
Currently, one of the large families of techniques of marking a digital document with a digital image is that of spread spectrum, another is that formed by the digital watermarking techniques with lateral information in the coder. A particular characteristic of techniques of spread spectrum digital watermarking in blind schemes is that they undergo the interference of the host itself. In contrast, the techniques with lateral information in the coder do not suffer said interference. Since, as previously indicated, the authenticity of the images of a video surveillance system needs to be blind, the most advanced schemes use techniques with lateral information in the coder.
An example of a technique for marking a digital document with a digital image with lateral information in the coder and blind extraction can be found in the article by B. Chen and G. W. Womell: “Quantization Index Modulation: A Class of Provably Good Methods for Marking a digital document with a digital image and Information Embedding”, published in IEEE Transactions on Information Theory, Vol. 47, No. 4, May 2001. This document shows the possibility of using quantizers to embed information in the host. Basically, the idea proposed is to have a set of quantizers of which one is selected depending on the message one wants to embed. The establishment of that set of quantizers is not trivial. The authors propose a practical procedure to efficiently establish them in a structured manner. Starting from a prototype quantizer, the reconstruction points are displaced, with the effect of establishing a different quantizer; this technique is called Dither modulation. On the other hand, it shows how it is possible to increase the robustness by lowering the transmission rate. One of the processes stated consists of mapping the values of the host before inserting the mark, in this way the noise that is orthogonal to the vector onto which it is mapped will not influence the communication. This document presents the technique called Distortion-Compensated (DC); hereby, it is possible to control the difference between the document marked and the original document. As a result, there is another value whereby a valid solution can be reached between the robustness of the watermark and the imperceptibility.
Another approach of the mapping in the marking of a digital document with a digital image can be found in the article by Fernando Pérez-González, Félix Balado, and Juan R. Hernández: “Performance analysis of existing and new methods for data hiding with known-host information in additive channels,” published in IEEE Transactions on Signal Processing, 51(4):960-980, April 2003. Special Issue on Signal Processing for Data Hiding in Digital Media & Secure Content Delivery. In this document, the authors give a wider vision of the mapping of the host values, as they reach a solution of compromise between the insertion techniques with lateral information in the coder and the spread spectrum techniques.
Another practical implementation of the techniques based on quantizers is found in the document by Joachim J. Eggers, Robert Bäuml, Tomas Tzschoppe and Bernd Girod: “Scalar Costa Scheme for Information Embedding”, published in IEEE Transactions on Signal Processing, VOL. 51, NO. 4, April 2003. This document shows a technique close to Dither modulation, but exclusively centred on scalar quantizers.
The previous articles presented have the common denominator of the theoretical approach of the watermarking schemes proposed. An example of this is to model the noisy communication channel as a channel with additive Gaussian white noise, while on many occasions the channel is characterized in that it has a quantization noise; as is the case of the coding of still images in JPEG or videos in MPEG-1.
There are several patents centred on the field of marking a digital document with a digital image for image authentication. Patent US2004131184 has the purpose of demonstrating the validity of videos to be used as irrefutable evidence in the justice system. It uses Dither-QIM watermarking techniques, introducing two types of information: one is of identity and the other control. The identity information is used to identify the video sequence, and that of control is used to determine if the image was manipulated. Another basic characteristic in this patent is that it only mentions the MPEG standard. That standard divides the coefficients of every block by a quantization matrix, therefore, there are large distortions in the information inserted at the time of compression of the group of images; as a consequence, to enter the information it is necessary to alter a large quantity of coefficients for every block. By marking a high number of coefficients, the difference between the original image and the image with the mark is, normally, quite perceptible. It is devised to be implemented in a laptop which accompanies the recording systems of police cars.
Patent EP1001604 shows a method for embedding information in images. It operates with still images coded with the JPEG or JPEG2000 standard, and an adaptation of SCS (Scalar Costa Scheme of Eggers et al.) is used to embed the information. It fixes the values of the size of the quantization steps used to embed the information, therefore reducing the versatility of the original method. Furthermore, it does not include any technique which permits embedding the information with a greater degree of robustness, such as mapping techniques.
A patent used to authenticate image flows is US2003172275, with the objective of guaranteeing copyright. It classifies the images which form in the flow as synchronous and asynchronous. In the synchronous images a mark is embedded in the blocks selected pseudo-randomly. It uses techniques of insertion with lateral information in the coder in the information introduction techniques. Due to the fact that the mark is not embedded in the whole image, it is not possible to locate the alterations.
An idea for combining the techniques of watermarking and network cameras, network camera servers or digital video servers is shown in patent US2004071311. It indicates a possible solution for integrating the cameras and the watermark embedding process from a physical standpoint. There are currently numerous manufacturers of network cameras, for which reason it is more feasible to design a method which perfectly adapts to the already existing cameras than try to design them from the start. The process to embed the watermark is characterized in that a robust watermark is inserted to be able to demonstrate the authenticity and another fragile watermark to locate the alterations. However, the patent does not disclose a method sufficiently complete so as to be able to tackle the problems typical of compression in JPEG or in any of the MPEG standards.
From the above, it is gathered that there is a need to find a practical solution for the problem of the recognition and location of spatial and/or time manipulations in still images or flows of images generated by digital video surveillance systems. Where said solution should provide a high degree of reliability and security, so that what the images show is irrefutable. Another necessary requirement, which has not yet been resolved, is the perfect adaptation of the authentication methods to the special characteristics of the existing digital video surveillance systems, such as resistance to the transcoding of JPEG to MPEG or the adaptability to the computational limitations of the devices that integrate those systems, for example: digital security cameras.